ASTRO: Question about skyglow

Question to all astroimagers about skyglow - specifically light
pollution spectra.

As I am ready to invest in a high cost CCD system, I would like to
know...

1 - How much skyglow are you working under?
(mag limit or SQM readings on average clear nights)

2 - What filters do you use for your imaging?
(type and make)

3 - Did you ever record, image, discover or research the skyglow
spectra you're working under and ideally should be removing?
(Yes or no would be fine).

George L wrote:

Question to all astroimagers about skyglow - specifically light
pollution spectra.

As I am ready to invest in a high cost CCD system, I would like to
know...

1 - How much skyglow are you working under?
(mag limit or SQM readings on average clear nights)

2 - What filters do you use for your imaging?
(type and make)

3 - Did you ever record, image, discover or research the skyglow
spectra you're working under and ideally should be removing?
(Yes or no would be fine).

1: Zenith magnitude is 6.5. Airglow is my limiting factor. Its rather
strong at my latitude. I have virtually no skyglow however.

2: When I worked from inside a town I found no filter worthwhile that
was labeled light pollution or something similar. The glow is just too
broadband to filter out without also filtering out your subject. But
there are solutions.

One is to use narrow band filters. That is, specialize in emission
nebula and use H alpha, OIII and SII filters for instance for green blue
and red respectively. That's what Richard Crisp does from Silicon Valley.

But with lots of exposure time (not in one exposure but lots of 5 minute
ones or so) you can build an image on the CCD that allows you to do
quite well. Takes a lot of image processing skills to get rid of all
the gradients caused by sky glow. It's anything but even across the
image so fun to deal with. Look at Stefan Lilge's posts from the middle
of Berlin. He does amazing work without filters from there but his
exposure times are long. 68 five minute frames for his post of NGC
3198. From my dark site I'd use about 4 or maybe 8 10 minute ones
depending on how bright it is. Think that one would work well at no
more than 5. Color LRGB images are more difficult as you have to deal
with the color of the skyglow as well as its intensity or you end up
with a green sky in most cases. Why a town full of high pressure orange
sodium lights turns the sky green I've never understood but that's what
it did when I tried imaging from in town.

For black and white from in town I got good results using a deep red
filter such as Lumicon's so-called Hydrogen Alpha filter. Though a
Wratten 25 was nearly as effective. It allowed me to do a lot of film
work that would have been impossible otherwise.

3: Nope. Since you can't really filter it due to its broadband nature I
never felt it worth the effort. Might explain the green sky problem but
won't help eliminate it.

Rick



--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

On Mon, 25 Feb 2008 01:09:06 -0600, Rick Johnson
wrote:

1: Zenith magnitude is 6.5. Airglow is my limiting factor. Its rather
strong at my latitude. I have virtually no skyglow however.

I'm very curious Rick, approximately where are you located? No need to
be specific, just give me a distance to nearest town of 1000 and city
of over 1million. Thanks for your reply.

About 12 miles from 95N 47N

Nearest town that's really one is Nevis at about 250 people and 13 miles.

Nearest town over 1000 is Park Rapids at about 3000

Both those are far smaller in winter than the official populations.
Most who can flee south.

Nearest over a million would be the metro Twin Cities area though no one
town there comes all that close to 1 million all together are about
that. That would be over 200 miles. I can't see any hint of it.

The township I live in (I'm near the center) is 36 square miles and has
22 voting residents of which only 9 are around this time of the year. I
can see not one yard light or home from my location. Since it is dense
forest most lighting never reaches the sky, towns do of course.

Where we hold the Nebraska Star Party SW of Valentine Nebraska the skies
are much darker and you see a rather strong shadow from the Milky Way.
Some claim 7.5 but I only reach 6.9 but my eyes are in their 7th decade
so not quite up to snuff any more.

I've attached a mid summer shot from the walls of the observatory. As
you can see there's not one sole out there. Lights from fishing boats
are about as bright as it gets. I once had a fisherman hit the
observatory with an intense spotlight trying to figure out what it was.
Fortunately it was cloudy so I wasn't imaging. Most of the land is
either Paul Bunyon State Forest or owned by 3M under a conservatorship
with the DNR to keep it wild. So there won't be anyone out there either.

Rick


George L wrote:

On Mon, 25 Feb 2008 01:09:06 -0600, Rick Johnson
wrote:

1: Zenith magnitude is 6.5. Airglow is my limiting factor. Its rather
strong at my latitude. I have virtually no skyglow however.


I'm very curious Rick, approximately where are you located? No need to
be specific, just give me a distance to nearest town of 1000 and city
of over 1million. Thanks for your reply.

George,

1 - I am imaging from the central area of Berlin. I don't "believe" in
limiting magnitudes as their estimation is as much dependend on one's
eyesight as on the quality of the sky. I can see about mag 4.5 stars near
zenith in a good night, but I know people who have better skies than myself
who insist that they have "mag 3 skies at best".

2 - I use Astronomik Halpha, UHC and OIII filters for emission line objects.
As Rick already mentioned, light pollution filters are usually useless for
broadband sources like galaxies or star clusters. I have tried "Astronomik
CLS", "Lumicon Deep Sky" and "Baader Skyglow" filters for galaxies and under
my city skies all of them cut the light of galaxies by about the same amount
as the light pollution, so there is no gain in contrast. On the other hand I
have been able to almost completely block the light of a row of streetlights
in a town in the northeast of Germany, so if your light pollution is mainly
caused by one specific source (e.g. certain type of streetlights) it might
be worth a try with some light pollution filters.

3 - I did not research the spectrum of my light pollution, but given that it
is a mix of all kinds of sources and given that light pollution filters
don't help here, I am quite sure that my light pollution is "broadband" and
not confined to certain regions of the spectrum.

Stefan


"George L" schrieb im Newsbeitrag
...
Question to all astroimagers about skyglow - specifically light
pollution spectra.

As I am ready to invest in a high cost CCD system, I would like to
know...

1 - How much skyglow are you working under?
(mag limit or SQM readings on average clear nights)

2 - What filters do you use for your imaging?
(type and make)

3 - Did you ever record, image, discover or research the skyglow
spectra you're working under and ideally should be removing?
(Yes or no would be fine).

Thanks for all the detailed information.

On Mon, 25 Feb 2008 20:48:13 -0600, Rick Johnson
wrote:

About 12 miles from 95N 47N

Nearest town that's really one is Nevis at about 250 people and 13 miles.

Nearest town over 1000 is Park Rapids at about 3000

Both those are far smaller in winter than the official populations.
Most who can flee south.

Nearest over a million would be the metro Twin Cities area though no one
town there comes all that close to 1 million all together are about
that. That would be over 200 miles. I can't see any hint of it.

The township I live in (I'm near the center) is 36 square miles and has
22 voting residents of which only 9 are around this time of the year. I
can see not one yard light or home from my location. Since it is dense
forest most lighting never reaches the sky, towns do of course.

Where we hold the Nebraska Star Party SW of Valentine Nebraska the skies
are much darker and you see a rather strong shadow from the Milky Way.
Some claim 7.5 but I only reach 6.9 but my eyes are in their 7th decade
so not quite up to snuff any more.

I've attached a mid summer shot from the walls of the observatory. As
you can see there's not one sole out there. Lights from fishing boats
are about as bright as it gets. I once had a fisherman hit the
observatory with an intense spotlight trying to figure out what it was.
Fortunately it was cloudy so I wasn't imaging. Most of the land is
either Paul Bunyon State Forest or owned by 3M under a conservatorship
with the DNR to keep it wild. So there won't be anyone out there either.

Rick


George L wrote:

On Mon, 25 Feb 2008 01:09:06 -0600, Rick Johnson
wrote:

1: Zenith magnitude is 6.5. Airglow is my limiting factor. Its rather
strong at my latitude. I have virtually no skyglow however.


I'm very curious Rick, approximately where are you located? No need to
be specific, just give me a distance to nearest town of 1000 and city
of over 1million. Thanks for your reply.

Thanks for replying, see below...

On Tue, 26 Feb 2008 20:48:50 +0100, "Stefan Lilge"
wrote:

George,

1 - I am imaging from the central area of Berlin. I don't "believe" in
limiting magnitudes as their estimation is as much dependend on one's
eyesight as on the quality of the sky. I can see about mag 4.5 stars near
zenith in a good night, but I know people who have better skies than myself
who insist that they have "mag 3 skies at best".

Some people don't bother to wear eyeglasses, like myself during the
day. But for star-gazing I gain at least 1-1/4 magnitudes over my
naked eyes upon wearing glasses.

2 - I use Astronomik Halpha, UHC and OIII filters for emission line objects.
As Rick already mentioned, light pollution filters are usually useless for
broadband sources like galaxies or star clusters. I have tried "Astronomik
CLS", "Lumicon Deep Sky" and "Baader Skyglow" filters for galaxies and under
my city skies all of them cut the light of galaxies by about the same amount
as the light pollution, so there is no gain in contrast. On the other hand I
have been able to almost completely block the light of a row of streetlights
in a town in the northeast of Germany, so if your light pollution is mainly
caused by one specific source (e.g. certain type of streetlights) it might
be worth a try with some light pollution filters.

Can anyone confirm by some experiment that their "light pollution is
mainly caused by one specific source"?

3 - I did not research the spectrum of my light pollution, but given that it
is a mix of all kinds of sources and given that light pollution filters
don't help here, I am quite sure that my light pollution is "broadband" and
not confined to certain regions of the spectrum.

Stefan

Very well. A conclusion that you cannot avoid. But if I told you can
research the info, at a minimum, and at the other extreme you can
obtain the info on your own..would you want to know? In other words,
is there a desire to know?

Oops that was obviously 95W

Hard to reach 95N!

Rick


George L wrote:
Thanks for all the detailed information.

On Mon, 25 Feb 2008 20:48:13 -0600, Rick Johnson
wrote:


About 12 miles from 95N 47N

Nearest town that's really one is Nevis at about 250 people and 13 miles.

Nearest town over 1000 is Park Rapids at about 3000

Both those are far smaller in winter than the official populations.
Most who can flee south.

Nearest over a million would be the metro Twin Cities area though no one
town there comes all that close to 1 million all together are about
that. That would be over 200 miles. I can't see any hint of it.

The township I live in (I'm near the center) is 36 square miles and has
22 voting residents of which only 9 are around this time of the year. I
can see not one yard light or home from my location. Since it is dense
forest most lighting never reaches the sky, towns do of course.

Where we hold the Nebraska Star Party SW of Valentine Nebraska the skies
are much darker and you see a rather strong shadow from the Milky Way.
Some claim 7.5 but I only reach 6.9 but my eyes are in their 7th decade
so not quite up to snuff any more.

I've attached a mid summer shot from the walls of the observatory. As
you can see there's not one sole out there. Lights from fishing boats
are about as bright as it gets. I once had a fisherman hit the
observatory with an intense spotlight trying to figure out what it was.
Fortunately it was cloudy so I wasn't imaging. Most of the land is
either Paul Bunyon State Forest or owned by 3M under a conservatorship
with the DNR to keep it wild. So there won't be anyone out there either.

Rick


George L wrote:


On Mon, 25 Feb 2008 01:09:06 -0600, Rick Johnson
wrote:


1: Zenith magnitude is 6.5. Airglow is my limiting factor. Its rather
strong at my latitude. I have virtually no skyglow however.


I'm very curious Rick, approximately where are you located? No need to
be specific, just give me a distance to nearest town of 1000 and city
of over 1million. Thanks for your reply.



--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".